This invention relates to a stripline filter suited for utilization in small-sized electric circuits.
In general, a stripline filter includes a pair of opposing, dielectric substrates each having an outer surface provided with a ground conductor, and conducting resonator fingers of a stripline pattern provided between the dielectric substrates and each having an open circuit end and a base end electrically connected to the ground conductor. Such a filter is utilized as a bandpass filter in the microwave frequency region.
The response characteristics of such stripline filters depend on the size of the resonator fingers and the shape of the dielectric substrates. Thus, a variation in such a size and a shape will cause a variation in coupling capacity of the filter so that the frequency to which the filter responds deviates from a predetermined frequency range.
The coupling capacity on the open circuit end side of each of the resonator fingers depends on the aperture distance between the open circuit end and the opposing ground conductor. Thus, as long as the length of the space between the resonator finger and the opposing ground conductor is not varied, the coupling capacity on the side of the open circuit end is maintained constant. For example, in the case of a stripline filter of an apron type in which, as shown in FIG. 2 of the accompanying drawings which will be described hereinafter, resonant fingers are connected to apron conductor portions 6e and 6f formed on the periphery of the dielectric substrate, the coupling capacity may be made constant by setting the distance between the open circuit ends and the apron conductor portions to a predetermined value.
However, the resonator fingers differ in shape from each other. In the case of a resonator with three stripline fingers, for example, the fingers of both sides have output and input terminals while the center finger has no such terminal. As a result, the length of the fingers should be separately determined with consideration of their electrical connection modes. In general, the center finger should be slightly longer than the fingers on the both sides. Since the apron conductor portions are those to which the base ends of fingers are connected and, hence, serve as basal portions to determine the length of each finger, the length of the opposing apron conductor portions is not to be varied. Thus, it is not possible to regularize the coupling capacities on the side of the open circuit ends of the resonator fingers by controlling the distance between the apron conductor portions. In the conventional stripline filters, therefore, the coupling capacities on the open circuit end sides are not uniform and it is necessary to trim the resonance frequency characteristics after fabrication thereof.
On the other hand, in the dielectric filter, the conductors are exposed on the outer surface of dielectric substrates. Considering from this fact, the following disadvantages are induced. The first is that the conductor is easily to be oxidized; the second is that this is easy to be abraded or ablated from the outer surface of the substrates; the third is that, since it is necessary to avoid the unnecessary electrical contact with the conductor, the use is not easy.